1. Field of the Invention
The present invention relates to a method of controlling communication, and particularly to a method of controlling communication in a communication system including a RFID tag and a reader/writer, which writes and reads information onto and from the RFID tag via wireless communication.
2. Description of the Related Art
As disclosed, for example, in Japanese Unexamined Patent Publication Nos. 7 (1995)-200749 (hereinafter, Patent Document 1) and 2008-033905 (hereinafter, Patent Document 2), a communication system including a RFID tag having a function to record information and a function to carry out wireless communication with respect to the information with an external device, and a reader/writer, which writes and reads information onto and from the RFID tag via wireless communication, has been known. Recently, this type of communication systems have been widely applied to automatic ticket gate systems at stations, merchandise management systems, etc. Further, as disclosed, for example, in Japanese Unexamined Patent Publication No. 2009-234599 (hereinafter, Patent Document 3), this type of communication systems are being applied to management of parts forming various devices and consumable supplies.
The communication systems including the RFID tag and the reader/writer generally use, as the communication method, FM modulation and/or AM modulation. For example, according to the ISO 15693-2 standard, which specifies a standard for this types of communication systems, both the FM modulation and the AM modulation are usable for transmission from the RFID tag to the reader/writer.
According to this standard, the FM modulation uses a combination of eight subcarrier pulses with a frequency of 423.75 kHz and nine subcarrier pulses with a frequency of 484.28 kHz to represent one bit That is, when these subcarriers are sent in this order, they represent “0”, and when these subcarriers are sent in reverse order, they represent “1”. In general, these subcarriers are referred to as a dual subcarrier.
On the other hand, the AM modulation uses a combination of a state when a pulsed subcarrier with a constant frequency rises in a predetermined period and a state when the pulsed subcarrier does not rise in the predetermined period to represent one bit That is, when these states are set in this order, they represent “0”, and when these states are set in reverse order, they represent “1”. In general, this subcarrier with a constant frequency is referred to as a single subcarrier.
In the communication systems using the RFID tag as described above, it is very difficult to completely prevent occurrence of an error in communication from the RFID tag to the reader/writer. Conventionally, in order to minimize occurrence of such a communication error, it is considered to carry out optimization of the modulation method and/or signal cancelling at the RFID tag to stabilize communication. However, at the reader/writer, transmission/reception with the RFID tag is simply controlled and the received signal is decoded without any additional processing. Therefore, even if an answer signal from the RFID tag has been sent back, an error may occur during decoding, resulting in unsuccessful reading of the data sent from the RFID tag.